In mobile communication technologies like, e.g. UMTS (Universal Mobile Telecommunication System), base stations—so-called first type network devices—serve a limited number of mobile users—so-called second type network devices—according to the current location of the users. As long as a user is in a cell area of certain base station, he can obtain mobile services from that base station. The overall performance and the quality of the service depends—among others—on propagation conditions, cell type, cell size, load distribution and on the power level of the various signal transmissions, particularly of the pilot signal provided by each base station.
The pilot signal transmitted by each base station carries a bit sequence or code known by the mobile stations. The bit sequence can be base station and sector dependent. The power level of the pilot signal received by the mobiles is used by the mobile stations to measure the relative distance between different base stations that could be used for communication. Thus, the power level of the pilot signal of a base station determines how far a mobile can “hear” the base stations; i.e. the power of the pilot signal is an indication to the mobile station of its ability to successfully use the signal from that base station which is transmitting that pilot signal.
In Code Division Multiple Access networks (WCDMA-Systems for example) the cell selection, re-selection and the selection of the active set of cells which are used for communication is based on the relative strength of the received pilot signal power (CPICH Ec/Io, wherein Ec/Io=chip energy to total interference spectral density) from different cells. Thus, the borders of a cell are determined by the relative strength of the pilot signal received from different cells. Hence, the power level of the pilot signal determines the pilot power coverage, i.e. the area of the cell in which the pilot signal is sufficiently powered to be properly decoded by the mobile stations.
In the CDMA-Systems tight and fast power control is an important aspect, particularly on the uplink channel to avoid that one single overpowered mobile station blocks a whole cell. The solution is a fast closed-loop power control. By this control, the base station performs frequent estimates of the received Signal to Interference Ratio (SIR) and compares it to a target SIR. If the measured SIR is higher than the target SIR, the base station will command the mobile station to lower the power. If it is too low, it will command the mobile station to increase its power. The closed loop power control will thus prevent power inbalance among all the uplink signals received at the respective base station.
Additionally, a slower Outer Loop Power Control (OLPC-function) is provided which adjusts the target SIR in the base station according to the needs of the individual radio link and aims at a constant target quality, usually defined as a bit error rate (BER) or block error rate (BLER). As one would waste much power capacity if one would set the target SIR to the worst case, i.e. for high mobile speeds, the target SIR floats around a minimum value that just fulfils the required target quality. The target SIR will change as a function of time, speed and the propagation environment of the mobile changes. When the Outer Loop Power Control adjusts the target SIR in the respective base station, the fast closed loop control will react correspondingly and bring the SIR-value received in the associated base stations back to the target SIR value.
During a Soft Handover situation (SHO) a mobile station is in the overlapping cell coverage area of two cells belonging to different base stations. The communications between mobile station and the base stations take place concurrently via two air interface channels downlink from each base station separately. In uplink direction, the code channel of the mobile station is received from both base stations, but the received data is then routed to the associated radio network controller (RNC) for combining. Then, the RNC selects the better communication between the two possible radio links, and this selection takes place periodically, i.e. every 10 to 80 milliseconds.
In WCDMA-systems base stations are not synchronized, which is different to other current CDMA technologies and the synchronization of a new radio link between a mobile station and the base station takes place during the radio link set up procedure. The uplink synchronization will be achieved and maintained only if the base station can receive a strong enough signal from the mobile, i.e. if the transmission power of the mobile station is high enough. The transmission power of the mobile, is however, controlled by the power control of the “stronger” radio link. In this application the stronger link is defined as radio connection requiring less transmission power from the mobile, and the weaker link is defined as radio connection requiring more transmission power from the mobile. The “stronger” cell is defined to be the cell with the stronger link, and the “weaker” cell is defined to be the cell with the weaker link.
If the links in a soft handover area are strongly asymmetric, the transmission power of the mobile station may not be high enough to reach the other base station(s) where the link is weaker. This implies that for the cell included in the active set of cells and requiring significantly higher transmission power from the mobile, the uplink synchronisation will not be achieved or maintained, and the link set up to establish a soft handover procedure or the maintaining of the soft handover will practically fail.
This applies to any communication system where the transmission of a second type network devices (e.g. mobile) is received by more than one first type network device (e.g. base station) to setup and/or maintain a communication link but its transmission power cannot be adjusted individually to each first type network devices.